1. Field of Invention
The present invention relates to radioactivity well logging.
2. Description of Prior Art
In the type of radioactivity well logging, known as chlorine logging, such as described in U.S. Pat. No. 3,772,513 which is assigned to the assignee of the present invention to distinguish oil zones from water zones in earth formations, a continuous neutron source is used to emit neutrons into the borehole and earth formations adjacent the borehole. A sleeve of samarium or other suitable material encloses the detector portion of the sonde containing the neutron source and generates selected energy gamma rays to compensate for the adverse effects of boron in the shale formations on the logging measurements. This type of logging is sensitive to borehole conditions, such as changes in borehole fluid salinity, borehole size, casing size, or the condition of the cement annulus. Large variation in any of these borehole factors could introduce errors in interpretation of the log.
It would thus be desirable to use a pulsed neutron source in chlorine logging and so to time gate out borehole effects. However, the time delays before the thermal neutron capture gamma rays are sensed, are sufficiently long that the selected energy gamma radiation component detected due to thermal neutron capture in the samarium sleeve is also substantially eliminated, thereby eliminating the shale or boron compensation provided by the samarium sleeve as described in the aforementioned patent.